Horses, Allergies, and Histamines
For the past year, I’ve spent a lot of time down the rabbit hole, examining the Western and Eastern medicinal approaches to horse allergies and equine asthma. As with other complicated health issues, the more I studied, the more research I followed, the more I realized there is a lot that isn’t known yet—particularly as it pertains to horses.
Nonetheless, it’s become apparent that more and more horses are dealing with allergy sensitivities and respiratory issues, and that allergies and equine asthma have become a concern for many horse owners.
What we DO know:
• Horses with asthma experience an imbalance between the chemical reduction and oxidation reactions in the body (or “redox”). The oxidation state is what is present in asthma.
• Equine asthma is associated with decreased antioxidant defenses, such as superoxide dismutase (AKA SOD, an enzyme that helps break down potentially harmful oxygen molecules in cells).
• The physiological roots of allergies are found in the inflammatory process and from oxidative stress.
• Allergies begin with the immune system. Allergic reactions such as hives and food allergies are the result of overstimulation of the immune system in response to a substance, such as pollen or a specific food.
The fascinating world of histamines:
When we think of allergies in equine, human, and canine we often immediately think of antihistamines. But what exactly are histamines?
Histamines are inflammatory mediators. Other inflammatory mediators you might recognize are cytokines, prostaglandins, leukotrienes, and reactive oxygen species.
Histamine receptors are located in the central nervous system, heart, lungs, vasculature, sensory nerves, gastrointestinal smooth muscles, immune cells and the adrenal gland.
When the immune system interprets a harmless substance as an invading pathogen, it releases histamines. These histamines bind to specific receptors throughout the body. Symptoms of histamines bound to receptors are runny nose, watery eyes, tissue swelling, constriction of bronchi.
Histamine release increases vasodilation, the dilation of blood vessels, which then increases the permeability of the blood vessel walls to help immune cells reach the affected area. This process is meant to help rid the body of foreign invaders, but it can be harmful in severe allergic reactions and chronic inflammatory diseases.
Histamines: friend or foe?
Histamines are important compounds, helping regulate stomach acid for digestion, and responding to inflammation. Histamines are part of the body’s immunoregulation function of the immune system.
According to a study published in 2018, “Histamine shows a dichotomous nature, whereby it is able to promote inflammatory and regulatory responses that contribute to pathological processes, such as allergy induction, as well as homeostatic functions, such as intestinal regulation.”1
Histamine and inflammation:
The role histamine plays in inflammation as a characteristic of equine asthma and allergies is sizable.
Histamines in the inflammatory process are released by mast cells as an immune response. Mast cells are multifunctional tissue-dwelling cells and are a major producer of histamine.
Mast cells also synthesize prostaglandin PGE-2, leukotrienes, and cytokines, playing a major role in the body’s inflammatory response.
Histamines and the Equine Stomach:
Because of the sensitivity of the equine GI tract, the role of histamine in this region of the body deserves notice. Histamine is believed to impact at least 3 major gut functions: modulation of GI motility, enhancement of gastric acid production, and alteration of mucosal ion secretion.2
Histamine is the primary stimulant of hydrochloric acid secretion in the stomach, which is essential for the gastric stage of digestion. Secondary stimulants are acetylcholine and gastrin.
Histamine and ulcer treatments:
HR2 blockers (also known as H2-receptor antagonists) are commonly used in the treatment and prevention of gastric ulcers. These receptor blocker medicines are known by various names: cimetidine, famotidine and ranitidine. These medications block the histamine receptors in order to decrease the amount of acid produced in the stomach.
Proton pump inhibitors, such as omeprazole (Gastro-Gard), inhibit the final stage of gastric acid secretion. Proton pump inhibitors are very effective at inhibiting histamine, acetylcholine and gastrin.
Histamine release in the large intestine:
Histamine is also released from mast cells during an inflammatory response, and can increase secretion and impair absorption by the colonic mucosa.3
Antihistamines and side effects:
Antihistamines are medications that are intended to reduce or block the effects of one of the four histamine receptors in the body.
Antihistamines are “H1 blockers” or “H2 blockers.” H1 blockers (like Benadryl, Chlor-Trimeton, and Dimetapp) target smooth muscle and endothelial cells, and H2 blockers (like Ranitidine or Cimetidine) antagonize histamine activity in the stomach. H2 blockers can reduce the amount of acid produced in the gut, since hydrochloric acid is triggered by histamine.
Common side effects of antihistamine medications are sedation and decreased coordination. A few horses get very excited on antihistamines, others can colic or have a loss of appetite.
Corticosteroids such as dexamethasone, are not histamine blockers. Instead, corticosteroids are potent anti-inflammatory drugs, inhibiting prostaglandin synthesis, and Cox-1 and Cox-2 (which produce the prostaglandins that contribute to pain, fever, and inflammation). Corticosteroids should be taken with caution.
Nutritional support for allergies and asthma in horses:
Quercetin Phytosome is a flavonoid from the Japanese Pagoda Tree, studied for its antioxidant properties. Quercetin helps inhibit histamines released by mast cells, most often associated with inflammation. A review of quercetin research was published in Molecule, May 2016: “Quercetin’s anti-allergic mechanism of action through the inhibition of enzymes and inflammatory mediators has been extensively studied. It is well known that quercetin is an inhibitor of human mast cell activation through the inhibition of Ca2+ influx, histamine, leukotrienes and prostaglandin release.”4
Resveratrol sourced from fermented yeast has been studied for its anti-inflammatory and antioxidant activities. Current research shows that it may decrease the release of histamine.
A study published in Allergy Asthma Immunol Res (Resveratrol in Asthma: a French Paradox?) stated: “Although resveratrol is less potent compared to glucocorticoids, it appears to be more effective in suppressing inflammatory activity”5
Superoxide Dismutase (SOD) is an enzyme from melons that breaks down potentially harmful oxygen molecules known as Reactive Oxygen Species (ROS). SOD constitutes an important antioxidant defense against oxidative stress in the body.
According to the World Allergy Organization Journal: “The major enzymatic antioxidants of the lungs are SODs, catalase and glutathione peroxidases. Asthma is characterized by the loss of antioxidant activities.”6
Patented Boswellia Serrata Resin with Quince Fruit Extracts (Alviolife®) supports anti-inflammatory action, specifically inhibition of 5-lipoxygenase (5-LOX), specific to the respiratory tract. This novel plant-sourced extract of boswellia and quince has been granted a license based on clinical trials from Health Canada to support respiratory health.
It’s obvious that allergies and equine asthma are complicated horse health issues. Understanding the role of histamine is important to addressing and supporting horses with allergies and asthma.
Coming soon! Biostar’s support formula: Aller-X EQ